For electrical insulating oils used for purposes of insulation, cooling and the like of transformers, cables, breakers, capacitors and the like, there have been long used mineral insulating oils that are obtained by subjecting a heavy crude oil to vacuum distillation for separation into given fractions and refining by treatment with sulfuric acid, an alkali, water washing, white clay or the like, and synthetic compound-based insulating oils such as diphenyls, silicones, phthalic acid esters and the like.
However, mineral insulating oils not only have the problem on safety and the like because of their high inflammability, but also have the possibility that they becomes difficult in future use from the standpoint of energy and environmental problems.
On the other hand, synthetic compound-based insulating oils also have problems such as of high inflammability, expensiveness and the like. Especially, with phthalic acid esters, it has been pointed out that they are suspected of having the endocrine disrupting action.
It will be noted that although PCB has been once in use, they have serious problems on safety, toxicity, environmental pollution and the like and thus, its use for electric equipments has been prohibited.
To cope with this situation, it has been expected to put natural vegetable oils having excellent safety such as soybean oil, rape seed oil, castor oil and the like to practical use as an electrical insulating oil. However, where a vegetable oil is applied to apparatus of the type where the inside is cooled by convection of an electrical insulating oil such as, for example, a large-sized transformer, the vegetable oil is disadvantageous in that it is high in viscosity and pour point and poor in stability against oxygen and heat (Patent Document 1). Hence, where these vegetable oils are used as an electrical insulating oil, is usual practice is to mix with mineral or synthetic compound-based insulating oils.
However, the mixing with a mineral or synthetic compound-based insulating oil does no lead to a fundamental solution to the above problem.
In recent years, the use, as an electrical insulating oil, of lower alcohol esterified products of vegetable oils such as rape seed oil, corn oil, safflower oil and the like have been proposed (Patent Documents 2 to 4).
In this connection, however, the specific permittivities of these insulating oils are smaller than those of insulating papers employed in electric apparatus and are not in conformity with the insulating paper with respect to the specific permittivity, for which an electric field stress is concentrated on the oil, thus making it difficult to downsize an apparatus in view of the problem on insulation. Additionally, these insulating oils are high inflammability and still have the problem of unsatisfactory stability to oxygen or heat.
Accordingly, the performance of these insulating oils is unsatisfactory for use as an electrical insulating oil capable of solving a future energy problem.
In view of the above, there have been proposed esterified compounds between trimethylolpropane/pentaerythritol and fatty acids having 7 to 18 carbon atoms for use as an insulating oil that is low in inflammability and pour point and excellent in biodegradability (Patent Document 5). However, this compound has problems of high viscosity and a poor cooling characteristic.
The present applicant has already proposed, as an electrical insulating oil excellent in viscosity, fluidity, chemical stability and the like, esterified products of higher fatty acids having 8 to 20 carbon atoms and branched fatty, monovalent alcohols having 6 to 14 carbon atoms, and esterified products of palm oil-derived mixed fatty acids and/or soybean oil-derived mixed fatty acids and aliphatic monovalent alcohols having 1 to 5 carbon atoms or branched aliphatic, monovalent alcohols having 6 to 14 carbon atoms (Patent Document 6).
Although this type of electrical insulating oil is excellent in viscosity, fluidity, chemical stability and the like, the flash point is relatively low and there is some room for improvement in safety.
As stated hereinabove, there has never been known any electrical insulating oil that is well balanced in characteristics such as low inflammability and safety, low viscosity and an excellent cooling characteristic, good stability to oxygen and heat, a high specific permittivity, capability of miniaturizing a transformer, and safety to the human body and environment, and can be used in practice without a problem. Thus, further improvements and developments have been demanded.
Patent Document 1:                JP-A 61-260503        
Patent Document 2:                JP-A 9-259638        
Patent Document 3:                JP-A 11-306864        
Patent Document 4:                JP-A 2000-90740        
Patent Document 5:                JP-A 2004-273291        
Patent Document 6:                PCT Patent Publication No. WO 2005/022558        